In recent years, an attempt to impart high hydrophilicity to a surface of a product has been made, in order to preventing the surface of glass or mirror from fogging, or to automatically remove the stains, adsorbed to the surface of an outdoor building, by the aid of rain water.
Methods for imparting high hydrophilicity to a surface of a product are disclosed, for instance, in Japanese Patent Application Laid-Open (JP-A) No. S61-83106 and WO96/29375. These methods described there are all characterized in that an inorganic coated film containing a photo-semiconductor material, e.g., titanium oxide is formed on a surface of a product, and the photocatalytic action of the photo-semiconductor material is utilized.
The photocatalytic action of a photo-semiconductor material means the action of decomposing organic substances, etc. present there as the result of redox reaction which is produced on the surface of the photo-semiconductor material when the light (ultraviolet rays) with excitation wavelength (for example, 400 nm) is applied to the photo-semiconductor material. Organic substances are generally hydrophobic. Hence, when these organic substances are decomposed and removed, the surface of the photo-semiconductor material becomes hydrophilic. As a result, the contact angle of water to the surface of the coated film becomes low, and the surface of the coated film tends to get wet (familiar with water).
However, it takes a certain period of time to develop the photocatalytic action of the photo-semiconductor material after it is irradiated with ultraviolet rays. Therefore there are the problems of, for instance, that the coated surface does not become hydrophilic, and tends to fog and stain for a certain fixed time after the film is formed. Also, there is, for instance, the problem of that it takes a longer period of time to develop the photocatalytic action at the place that is scarcely irradiated with ultraviolet rays, and the use of the photo-semiconductor materials is limited to the applications at which ultraviolet rays is easily applied.